Plexifilamentary sheets

Abstract

A sheet having a plexifilamentary structure with a normalized Frazier air permeability of between 0.002 and 0.2 (m 3 /m 2 ·minute)@50 gsm, and a normalized hydrohead of between 150 and 250 centimeters@50 gsm.

We claim: 
     
       1. A sheet comprising a plexifilamentary film-fibril network consisting of high density polyethylene in an overlapping multi-directional configuration, the sheet, as-produced from a spin cell, having a normalized Frazier air permeability of between 0.002 and 0.2 m 3 /m 2 ·minute at 50 gsm, a BET surface area of between 9 and 20 m 2 /gm, and a normalized hydrohead of between 150 and 250 cm at 50 gsm wherein the plexifilamentary film-fibril network in an overlapping multi-directional configuration is flash-spun from a spin fluid comprising polymer and spin agent, the spin fluid temperature being 190° C. to 195° C.,
 the spin agent consisting of dichloromethane and 2,3-dihydrodecafluoropentane, and the polymer concentration in the spin fluid being 12 to 14 weight percent. 
 
     
     
       2. The sheet of  claim 1  wherein the sheet as-produced from the spin cell has a normalized Frazier air permeability of less than or equal to 0.09 m 3 /m 2 ·minute at 50 gsm. 
     
     
       3. The sheet of  claim 1  wherein the sheet as-produced from the spin cell has a Frazier air permeability of greater than or equal to 0.0075 m 3 /m 2 ·minute at 50 gsm. 
     
     
       4. The sheet of  claim 1  wherein the sheet as-produced from the spin cell has a normalized hydrohead of between 165 and 208 centimeters at 50 gsm. 
     
     
       5. The sheet of  claim 1  wherein the sheet as-produced from the spin cell has a basis weight of greater than or equal to 30 grams per square meter. 
     
     
       6. A multilayer structure comprising a multiplicity of two or more sheets wherein at least one sheet is a sheet according to  claim 1 . 
     
     
       7. A thermally consolidated sheet made from sheet according to  claim 1 . 
     
     
       8. A multilayer structure comprising a multiplicity of two or more sheets wherein at least one sheet is a sheet according to  claim 7 .